The present invention relates to a liquid crystal display apparatus and, in particular, to a reflection type liquid crystal display apparatus, which features low power consumption, is light in weight and has a reduced thickness.
In order to provide an electronics device having multi-functions which is able to operate smoothly, the provision of an interface device therefor is becoming more and more important. A liquid crystal display apparatus which has a reduced-thickness, is light in weight, has a low power consumption and can be installed without need of a large modification of the arrangement of its electronics device, will be most suitable as an interface.
Recently, in order to realize a low power consumption in devices, reflection-type color liquid crystal display apparatuses have become available on the market. Because these reflection-type color liquid crystal display apparatuses use external light to enhance their display by reflecting this external light, low power consumption can be expected. However, there is a problem in that, although a good display can be obtained in a bright environment, such as a sunny outdoor area or the like, the display produced by such an apparatus becomes hardly visible in a dark indoor environment with a dim illumination or the like where visibility is substantially reduced. More specifically, among such reflection-type color liquid crystal display devices, there are a mono-polarization type that uses a single polarization plate, a guest-host type that uses a dichromatic pigment-added liquid crystal layer, and others; however, the reflectance and the contrast ratio of any of these devices are lower than those of a printed color copy or the like, and their visibility tends to drop in a dark environment. Even if a reflection-type color liquid crystal display device having the same or better reflectance and contrast ratio as those of printed matter is realized, it is inevitable for its visibility to tend to drop in a darker environment.
Therefore, in order to improve the visibility of a produced display in a dark environment, provision of an auxiliary light source is essential in a reflection-type color liquid crystal display device. In a conventional reflection type color liquid crystal display device, an auxiliary light source, which is referred to as a front light, is provided. As viewed from the user, in an arrangement of a liquid crystal display device having a first substrate near to the user and a second substrate remote therefrom for retaining a liquid crystal layer therebetween, the front light is provided in the upper direction of the first substrate toward the user, and the source of this light is comprised of a fluorescent lamp and a light guide plate. The light guide plate is comprised of a material having a larger refractive index than that of air, and is disposed to surround the whole surface of the first substrate. While the bottom surface of the light guide is disposed in parallel with the first substrate, the upper surface thereof is inclined to form an interface with the air. The fluorescent lamp is disposed on the side of the light guide plate, and light emitted therefrom enters the light guide plate. A component of light which has entered into the light guide plate, and which propagates in parallel with the bottom surface of the light guide plate, reaches the upper surface of the light guide plate, then is reflected due to a difference in refractive indexes between the light guide plate and the air in a direction of the first substrate so as to illuminate the liquid crystal display portion.
Further, in the prior art reflection type color liquid crystal display device, an arrangement for enabling a color display is known wherein color filters are used in the same manner as in a transmission type color liquid crystal display apparatus. In this case, in order to improve its brightness when an auxiliary light source is not used, a hypochromic filter is used as the color filter in this reflection type color liquid crystal display device.
The above-mentioned front light is described, for example, in JPA Nos. 10-268308 and 10-268306.
Further, JPA No. 10-213799 discloses a liquid crystal display apparatus wherein a flat auxiliary light source which surrounds the whole area of the liquid crystal display is provided. This auxiliary light source is a transparent light source, and an example using electroluminescence elements is disclosed wherein a transparent electrode is used for its electrode. This auxiliary light source is disposed to surround the whole area of the first substrate, and its light of emission directly illuminates the liquid crystal display portion. Also, external light is allowed to reach the display portion, which light is transmitted through the transparent electrode of the auxiliary light source and its light emitting layer.
Because a photoconductor of the conventional front light is as thick as 3 mm or more, it fails to satisfy the features of reduced thickness and lightness in weight called for by the invention. Further, there is a problem in that, because the photoconductor of the front light reflects external light incident on its upper surface, the contrast ratio drops. Further, when it is attempted to manufacture a liquid crystal display device in combination with a touch panel, the contrast ratio will drop further due to an increased number of interfaces with the air.
As for the front light of this type of display device, although it is ideal for the light of a fluorescent lamp to propagate in parallel with the bottom surface of the light guide plate, there actually exists a component of light that does not propagate in such a manner. Further, reflection of light on the upper surface of the light guide plate utilizes a difference of refractive indexes between the light guide plate and the air, and, therefore, its reflectance is low. Therefore, the light utilization efficiency of the front light is not high. Thereby, in order to achieve a sufficient brightness, the intensity of the fluorescent lamp must be increased. Although use of the front light is limited mainly in a dark environment, if the power consumption is increased in order to increase the brightness of the fluorescent lamp, it becomes impossible to satisfy the desire for low power consumption in a reflective color liquid crystal display.
On the other hand, an auxiliary light source of a flat type, as disclosed in JPA-10-213799, which covers the whole surface of a liquid crystal display unit, emits light not only in the direction of its liquid crystal layer, but also in the direction of the user. Thereby, because the light advancing directly toward the user does not undergo modulation by the liquid crystal display unit, the contrast ratio of the display drops.
As described above, the auxiliary light of a conventional type has such problems that it detracts from the advantages of the reflection type color liquid crystal display apparatus of being thin, light-weight and having a low power consumption, not to mention the fact that its contrast ratio is reduced.
Further, because the reflection type color liquid crystal display device needs to use hypochromic color filters as described above, and because the hypochromic color filters are a color filter that has a smaller quantity of color pigments than usual ones, there is such a problem in that, although it features a higher light transmittance than the color filter for a transmission type color liquid crystal display apparatus, it has a lower color purity.
Generally, in the case of the reflection type color liquid crystal display device that uses color filters, the color purity of the display colors is determined by a product between a square of the transmission spectrum of its color filter and its emission spectrum. Therefore, because hypochromic filters are used in the reflection type color liquid crystal display device, when white light is used as a light source, the color purity drops substantially in comparison with the transmission type color liquid crystal display apparatus. In case the auxiliary light source is not turned on in the reflection type color liquid crystal display device, external light incident from the surrounding area becomes its light source, and because most of external light is white light, the color purity of the displayed colors is low. Further, because the conventional front light is white light, even when the auxiliary light is turned on, its color purity remains low.
An object of the present invention is to solve the above problems associated with conventional display devices and provide for a liquid crystal display apparatus that can achieve a higher contrast ratio notwithstanding the fact that it is a reflection type liquid crystal display device.
In order to achieve the above object, a novel liquid crystal display apparatus according to the invention is provided.
Namely, a liquid crystal display apparatus of the invention is comprised of a liquid crystal layer, upper and lower substrates that hold the liquid crystal layer therebetween, a reflection plate disposed beneath the liquid crystal layer for reflecting light emitted from above the liquid crystal layer and which is transmitted therethrough, and a light emitting element layer for emitting light to the liquid crystal layer, wherein the light emitting element layer is disposed above the liquid crystal layer in such a manner as to surround the display portion of the liquid crystal layer, and the light emitting element layer is provided with a reflecting layer for deflecting light emitted in an upper direction back to the liquid crystal layer.